1. Technical Field of the Invention
The present invention relates generally to a method of producing a spark plug to have an increased high temperature oxidation resistance and a weld strength and a spark plug produced thereby.
2. Background Art
Usually, spark plugs used in environmental conditions where they are subjected to heavy thermal loads have a ground electrode made of a Ni-based alloy such as Inconel (trade mark) exhibiting a higher temperature oxidation resistance in order to minimize the wear of the ground electrode arising from its internal progress of spark-caused oxidation.
Modern engines are progressing in lean burning, so that thermal load conditions in which an electrode of a spark plug is heated up to a higher temperature and cooled rapidly is becoming severe. It is presumed that the temperature of the ground electrode will reach an upper limit of a high temperature oxidation resistance of Inconel 600 in the near future, thereby resulting in a difficulty in controlling the spark-caused wear of the ground electrode, which will lead to a decrease in service life of spark plugs.
In order to increase the high temperature oxidation resistance of the ground electrode, use of material is proposed which forms a firm oxide film easily on the surface of the ground electrode at an initial stage of use of the spark plug to arrest the spread of oxidation into the inside of the ground electrode.
The use of such material is excellent in improving the high temperature oxidation resistance of the ground electrode, but however, results in formation of an oxide film on a joint of the ground electrode with a metal shell during welding in the air, thus resulting in an undesirable decrease in mechanical strength of the joint.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a production method of a spark plug which is designed to improve the high temperature oxidation resistance of a ground electrode without sacrificing the mechanical strength of a weld of the ground electrode with a metal shell and a spark plug produced thereby.
According to one aspect of the invention, there is provided a production method of a spark plug which may be employed in automotive internal combustion engines. The spark plug includes a metal shell, a center electrode retained within the metal shell through electric insulation, and a ground electrode opposed to the center electrode through a spark gap. The method comprises: the steps of: (a) preparing the ground electrode made of a Ni-based alloy containing 10% or more by weight of Cr and 1.5% or more by weight of Al; and (b) joining the ground electrode to the metal shell by resistance welding in a substantially oxygen free atmosphere. The results of research made by the inventors of this application showed that the use of the Ni-based alloy containing 10 Wt % or more of Cr and 1.5 Wt % or more of Al causes oxides of Cr and/or Al to be deposited on the surface of the ground electrode to form a firm oxide film thereon which arrests the spread of oxidization into the inside of the ground electrode, thereby minimizing the spark-caused wear of the ground electrode, that is, improving the high temperature oxidation resistance thereof.
Further, welding the ground electrode made of such a material to the metal shell in the air causes an oxide film to be formed easily on a weld interface, thus resulting in a great decrease in mechanical strength of the weld interface. This is avoided in this invention by welding the ground electrode to the metal shell in the substantially oxygen free atmosphere.
In the preferred mode of the invention, the ground electrode contains 20% or less by weight of Cr and 5% or less by weight of Al. This is because too much Cr and Al will result in an undesirable increase in hardness which causes a difficulty in drawing the material of the ground electrode.
The oxygen free atmosphere is provided by an inert gas such as Ar.
If a ratio of a surface area S of the ground electrode to a volume V of the ground electrode is defined as S/V, a relation of 1.8 mmxe2x88x921xe2x89xa6S/Vxe2x89xa63.9 mmxe2x88x921 is satisfied.
Usually, thickening the ground electrode causes the load required to bend the ground electrode to define a desired spark gap to be increased, which results in ease of breakage or rupture of the ground electrode at the weld interface. Conversely, thinning the ground electrode results in a drop in high temperature oxidation resistance of the ground electrode. The results of research performed by the inventors of this application showed that a relation of 1.8 mmxe2x88x921xe2x89xa6S/V alleviates the rupture of the ground electrode at the weld interface during adjustment of the spark gap, and a relation of S/Vxe2x89xa63.9 mmxe2x88x921 ensures a desired high temperature oxidation resistance of the ground electrode.
The resistance welding may be performed while applying a jet of given gas producing the oxygen free atmosphere to portions of the ground electrode and the metal shell to be joined. This allows spark plugs to be produced at low costs.
The flow rate of the jet of the given gas is preferably within a range of 2L to 30L per minute.
The use of such a jet usually causes a weld target to be cooled. Since the ground electrode is typically smaller in volume than the metal shell, the ground electrode is cooled rapidly, thus resulting in a decrease in weldability thereof. The results of research made by the inventors of this application showed that the use of a material containing Al in making the ground electrode results in an increase in resistivity thereof, which serves to increase the quantity of heat produced at the ground electrode, thus canceling an adverse effect of the application of the jet of gas to the ground electrode.
According to the second aspect of the invention, there is provided a spark plug which comprises: (a) a metal shell; (b) a center electrode retained within the metal shell through electric insulation; and (c) a ground electrode joined at an end thereof to the metal shell by resistance welding and opposed at the other end to the center electrode through a spark gap. The ground electrode contains 10% or more by weight of Cr and 1.5% by weight of Al. The number of aluminum oxide particles per 0.1 mm in length of a weld interface between the end of the ground electrode and the metal shell is one or less, thereby ensuring a desired mechanical strength of the weld interface.
The ground electrode may be made of a Ni-based alloy containing 20% or less by weight of Cr and 5% of less by weight of Al.